THE SHOP

Performance & Hotrod Business - January '15

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12 n PRECISION ENGINE n January 2015 PRECISION ENGINE will accordingly affect how the block is stressed, resulting in variations of cylin- der bore shape. As mentioned earlier, when measuring a bore for shape, we need to remember that, depending on the instrumentation being used, we may or may not obtain a true bore shape. If we use a bore gauge, we are simply measuring bore diameter (and out of round) in specific height levels of the bore, but since we are not referencing from the true bore centerline, we may be overlooking potential shifts of the center- line at various height locations. If in doubt, or if we know that the radius of the bore has shifted relative to the centerline, we can use a precision bore-truing fixture to correct the problem (such as those offered by BHJ, CWT, etc.), or take advantage of CNC technology in order to create a "new" bore centerline based on blueprint data from the block manufacturer. The bores can then be machined to an oversize, with the cutters referenced from a fixed X and Y correct centerline. This will create bore trueness in a static condition, but subsequent changes may still occur when the block is subjected to stress loads, pressures and temperature. Keep in mind that when we're dealing with cast iron or aluminum blocks, the very nature of the casting process and the If you plan an overbore, or are dealing with a block that has already been over-bored, do yourself a favor and sonic-check cylinder wall thickness to verify that each bore has enough meat to allow the planned oversize. Cylinder walls that are already too thin, or that will be too thin after boring, will have a tendency for excess distortion and may result in pinholes or cracks at water jacket areas. If walls are too thin, cylinder sleeving is a viable option (otherwise, you'll need another block). A CNC machine can map the desired bore centerline in preparation of the boring operation. CNC boring offers a high degree of precision in terms of correcting any bore centerline variances. If a CNC machining center isn't available, specialty accurizing fixtures may be used on a conventional boring bar. This also allows proper centering of each bore. (Photo courtesy BHJ) When installing studs, do not over-tighten! Install finger-tight with a very minor added snug. Studs do not need to be installed to the block at the same torque as bolts. Over-tightening studs can result in uneven block stress and slight splaying that will affect main cap or head installation.

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